US20190123713A1 - Acoustic wave device - Google Patents
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- US20190123713A1 US20190123713A1 US16/158,333 US201816158333A US2019123713A1 US 20190123713 A1 US20190123713 A1 US 20190123713A1 US 201816158333 A US201816158333 A US 201816158333A US 2019123713 A1 US2019123713 A1 US 2019123713A1
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- 239000000758 substrate Substances 0.000 claims abstract description 40
- 229910012463 LiTaO3 Inorganic materials 0.000 claims description 12
- 230000001902 propagating effect Effects 0.000 claims description 12
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- BPUBBGLMJRNUCC-UHFFFAOYSA-N oxygen(2-);tantalum(5+) Chemical compound [O-2].[O-2].[O-2].[O-2].[O-2].[Ta+5].[Ta+5] BPUBBGLMJRNUCC-UHFFFAOYSA-N 0.000 claims description 2
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- PBCFLUZVCVVTBY-UHFFFAOYSA-N tantalum pentoxide Inorganic materials O=[Ta](=O)O[Ta](=O)=O PBCFLUZVCVVTBY-UHFFFAOYSA-N 0.000 claims description 2
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Images
Classifications
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- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
- H03H9/02—Details
- H03H9/02535—Details of surface acoustic wave devices
- H03H9/02818—Means for compensation or elimination of undesirable effects
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- H03H9/02—Details
- H03H9/125—Driving means, e.g. electrodes, coils
- H03H9/145—Driving means, e.g. electrodes, coils for networks using surface acoustic waves
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- H03H9/02559—Characteristics of substrate, e.g. cutting angles of lithium niobate or lithium-tantalate substrates
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- H03H9/02—Details
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- H03H9/02543—Characteristics of substrate, e.g. cutting angles
- H03H9/02574—Characteristics of substrate, e.g. cutting angles of combined substrates, multilayered substrates, piezoelectrical layers on not-piezoelectrical substrate
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- H03H9/02818—Means for compensation or elimination of undesirable effects
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Definitions
- the present invention relates to an acoustic wave device including a multilayer substrate in which a high acoustic velocity material layer, a low acoustic velocity material layer, and a piezoelectric film are laminated.
- An IDT (interdigital transducer) electrode of an acoustic wave device includes first and second busbars opposing to each other, a plurality of first electrode fingers, and a plurality of second electrode fingers. One end of each of the first electrode fingers is connected to the first busbar. One end of each of the second electrode fingers is connected to the second busbar. Tips of the first electrode fingers are arranged with gaps interposed between the tips and the second busbar. Tips of the second electrode fingers are arranged with gaps interposed between the tips and the first busbar.
- Japanese Unexamined Patent Application Publication No. 2011-101350 discloses that wave guide is performed when a length of the gaps is 1 ⁇ or longer, and that the gap length is more desirably 3 ⁇ or longer.
- the gap between the tip of the electrode finger and the busbar desirably has a longer length.
- a propagation loss increases at a longer gap length in an acoustic wave device in which a piezoelectric substrate is a multilayer substrate.
- a propagation loss tends to increase in a multilayer substrate in which a high acoustic velocity material layer, a low acoustic velocity material layer, and a piezoelectric film are laminated.
- high acoustic velocity material denotes a material in which the acoustic velocity of a bulk wave propagating in the material is higher than the propagation velocity of an acoustic wave propagating in the piezoelectric film.
- low acoustic velocity material denotes a material in which the acoustic velocity of a bulk wave propagating in the material is lower than the propagation velocity of the bulk wave propagating in the piezoelectric film.
- Preferred embodiments of the present invention provide acoustic wave devices each including a multilayer substrate and having a reduced propagation loss.
- an acoustic wave device includes a multilayer substrate including a reverse-velocity surface (the inverse of SAW-phase velocity), the multilayer substrate including a piezoelectric film, a low acoustic velocity material layer in which a bulk wave having a lower velocity than an acoustic velocity of an acoustic wave propagating in the piezoelectric film propagates, and a high acoustic velocity material layer in which a bulk wave having higher velocity than the acoustic velocity of the acoustic wave propagating in the piezoelectric film propagates, the low acoustic velocity material layer being disposed between the high acoustic velocity material layer and the piezoelectric film; and an IDT electrode disposed on the piezoelectric film, wherein the IDT electrode includes first and second busbars opposing each other, a plurality of first electrode fingers extending from the first busbar toward the second busbar, and a plurality of second electrode fingers extending from
- an intersecting region in which the first electrode fingers and the second electrode fingers are overlapped with each other in a propagation direction of the acoustic wave includes a central region at a middle in the extension direction of the first and second electrode fingers, and first and second edge regions on both of the outer sides of the central region in the extension direction of the first and second electrode fingers, and the acoustic velocity in each of the first and second edge regions is lower than the acoustic velocity in the central region. In such a case, transverse mode ripples are reduced or prevented.
- widths of the first and second electrode fingers in the first and second edge regions are greater than widths of the first and second electrode fingers in the central region, the widths being measured in the propagation direction of the acoustic wave.
- each of the first and second busbars includes a plurality of openings arrayed side by side in the propagation direction of the acoustic wave, portions of the first and second busbars on a side closer to the intersecting region than the openings define and function as first and second thin busbar portions, and portions of the first and second busbars on a side farther away from the intersecting region than the openings define and function as first and second outer busbar portions, the first and second thin busbar portions being connected respectively to the first and second outer busbar portions by connecting portions between the adjacent openings.
- the piezoelectric film is made of LiTaO 3 and has a thickness of about 3.5 ⁇ or smaller. In such a case, a Q value is increased.
- the high acoustic velocity material layer may be a support substrate made of a high acoustic velocity material.
- the acoustic wave device may further include a support substrate that supports the high acoustic velocity material layer.
- first dielectric films that define mass adding films are laminated on the first and second edge regions.
- metal films that define mass adding films are laminated on portions of the first and second electrode fingers positioned in the first and second edge regions.
- the first dielectric films are further laminated on the first and second busbars and over the first and second gaps.
- a second dielectric film that defines a frequency adjustment film is laminated on the IDT electrode, and the first dielectric films are laminated with the second dielectric film interposed between the IDT electrode and the first dielectric films.
- propagation losses are able to be reduced or prevented in the acoustic wave devices including the multilayer substrate in which the high acoustic velocity material layer, the low acoustic velocity material layer, and the piezoelectric film are laminated.
- FIG. 1 is a partial plan view illustrating a principal portion of an electrode structure of an acoustic wave device according to a first preferred embodiment of the present invention.
- FIG. 2 is a schematic front sectional view of the acoustic wave device according to the first preferred embodiment of the present invention.
- FIG. 3 is a schematic plan view illustrating the electrode structure of the acoustic wave device according to the first preferred embodiment of the present invention.
- FIG. 4 is a partial enlarged plan view, partly cut out, illustrating a portion of FIG. 1 in an enlarged scale.
- FIG. 5 is an illustration representing reverse-velocity surfaces (the inverse number of SAW-phase velocity) of LiTaO 3 and a multilayer substrate included in the first preferred embodiment of the present invention.
- FIG. 6 is a graph representing return loss characteristics of the acoustic wave device in which a gap length G is about 0.17 ⁇ , about 0.28 ⁇ , or about 0.61 ⁇ .
- FIG. 7 is a graph representing a relationship between the gap length G in an IDT electrode and return loss.
- FIG. 8 is a schematic view to explain the reason why a propagation loss reduces in the acoustic wave device according to the first preferred embodiment of the present invention.
- FIG. 9 is a partial plan view, partially cut out, illustrating an electrode structure of an acoustic wave device according to a second preferred embodiment of the present invention, and acoustic velocities in various regions.
- FIG. 10 is a plan view to explain an electrode structure of an acoustic wave device according to a third preferred embodiment of the present invention.
- FIG. 11 is a plan view to explain an IDT electrode in an acoustic wave device according to a fourth preferred embodiment of the present invention.
- FIG. 12 is a plan view to explain an electrode structure of an IDT electrode in an acoustic wave device according to a fifth preferred embodiment of the present invention.
- FIG. 13 is a schematic side sectional view of an acoustic wave device according to a sixth preferred embodiment of the present invention.
- FIG. 14 is a plan view to explain an electrode structure of an IDT electrode in an acoustic wave device according to a seventh preferred embodiment of the present invention.
- FIG. 15 is a schematic side sectional view of the acoustic wave device according to the seventh preferred embodiment of the present invention.
- FIG. 16 is a schematic side sectional view of an acoustic wave device according to an eighth preferred embodiment of the present invention.
- FIG. 17 is a schematic side sectional view to explain a modification of the acoustic wave device according to the eighth preferred embodiment of the present invention.
- FIG. 18 is a schematic side sectional view to explain another modification of the acoustic wave device according to the eighth preferred embodiment of the present invention.
- FIG. 19 is a graph representing relationships between the length of an edge region and the electromechanical coupling coefficient in transverse modes in the acoustic wave device according to the fourth preferred embodiment of the present invention.
- FIG. 20 is a graph representing relationships between the length of an edge region and the electromechanical coupling coefficient in transverse modes in the acoustic wave device according to the seventh preferred embodiment of the present invention.
- FIG. 21 is a front sectional view of an acoustic wave device according to a ninth preferred embodiment of the present invention.
- FIG. 22 is a graph representing a relationship between a film thickness of LiTaO 3 and Q characteristics.
- FIG. 2 is a schematic front sectional view of an acoustic wave device 1 according to a first preferred embodiment of the present invention
- FIG. 3 is a schematic plan view illustrating an electrode structure of the acoustic wave device.
- the acoustic wave device 1 includes a multilayer substrate 2 .
- the multilayer substrate 2 includes a high acoustic velocity support substrate 3 defining a high acoustic velocity material layer, a low acoustic velocity material layer 4 , and a piezoelectric film 5 preferably made of LiTaO 3 , for example.
- the low acoustic velocity material layer 4 and the piezoelectric film 5 are laminated on the high acoustic velocity support substrate 3 in that order.
- An IDT electrode 6 and reflectors 7 and 8 are disposed on the piezoelectric film 5 .
- a one-port acoustic wave resonator is thus provided.
- a high acoustic velocity material defining the high acoustic velocity material layer is a material such that the acoustic velocity of a bulk wave propagating in the material is higher than the acoustic velocity of an acoustic wave propagating in the piezoelectric film 5 .
- a low acoustic velocity material defining the low acoustic velocity material layer 4 is a material such that the acoustic velocity of a bulk wave propagating in the material is lower than the acoustic velocity of the acoustic wave propagating in the piezoelectric film 5 .
- the multilayer substrate 2 including the low acoustic velocity material layer 4 and the piezoelectric film 5 laminated on the high acoustic velocity support substrate 3 energy of the acoustic wave is effectively confined inside the piezoelectric film 5 .
- the IDT electrode 6 includes first and second busbars 11 and 12 opposing each other. One end of each a plurality of first electrode fingers 13 is connected to the first busbar 11 . The first electrode fingers 13 extend toward the second busbar 12 . One end of each of a plurality of second electrode fingers 14 is connected to the second busbar 12 . The second electrode fingers 14 extend toward the first busbar 11 .
- a space between a tip of each first electrode finger 13 and the second busbar 12 is a first gap.
- a space between a tip of each second electrode finger 14 and the first busbar 11 is a second gap.
- Lengths of the first and second gaps in an extension direction of the first and second electrode fingers 13 and 14 are each referred to as a gap length G hereinafter.
- the gap length G is preferably set to about 0.23 ⁇ or shorter, for example, such that the propagation loss is able to be reduced.
- FIG. 3 schematically illustrates the IDT electrode 6
- FIGS. 1 and 4 illustrate a principal portion of the IDT electrode 6 in an enlarged scale.
- FIG. 4 further indicates the acoustic velocities in various regions.
- Each of the first electrode fingers 13 includes greater width portions 13 a and 13 b .
- the “width of an electrode finger” means the dimension of the electrode finger in the propagation direction of an acoustic wave.
- Each of the second electrode fingers 14 also includes greater width portions 14 a and 14 b .
- the greater width portions 13 a and 14 a are provided on the tip sides of the first and second electrode fingers 13 and 14 , respectively.
- the greater width portion 13 b is disposed at a position opposing the greater width portion 14 a in the propagation direction of the acoustic wave.
- the greater width portion 14 b is disposed in an opposing relationship to the greater width portion 13 a in the propagation direction of the acoustic wave.
- an intersecting region C i.e., a region at which the first electrode fingers 13 and the second electrode fingers 14 are overlapped with each other in the propagation direction of the acoustic wave, includes a central region C 1 , and first and second edge regions C 2 and C 3 on both the outer sides of the central region C 1 in a direction perpendicular or substantially perpendicular to the propagation direction of the acoustic wave. Furthermore, a space on the outer side of the first edge region C 2 provides a gap region at which the second gap is provided, the gap region being called a “second gap region”.
- FIG. 4 diagrammatically indicates the acoustic velocities in the above-described regions. Regarding the acoustic velocities in the above-described regions illustrated in FIG. 4 , the acoustic velocity is higher toward the right in FIG. 4 .
- the IDT electrode 6 is designed as follows.
- the acoustic velocity V 2 in the first and second edge regions C 2 and C 3 is lower than the acoustic velocity V 1 in the central region C 1 .
- the gap regions having the acoustic velocity V 3 are provided on the outer sides of the first and second edge regions C 2 and C 3 .
- the acoustic velocity in the regions at which the first and second busbars 11 and 12 are provided is V 4 that is lower than V 3 .
- Levels of the acoustic velocities V 1 to V 4 preferably have a relationship of V 3 >V 4 >V 1 >V 2 .
- transverse modes cause a problem.
- the IDT electrode 6 By designing the IDT electrode 6 to satisfy the above relationship among the acoustic velocities, the transverse modes are reduced or prevented due to the difference in the acoustic velocity. It is, however, to be noted that the present invention is not limited to acoustic wave devices including electrode structures that reduce or prevent the transverse modes.
- the acoustic wave device 1 includes the gap length G that is preferably about 0.23 ⁇ or shorter, for example.
- the gap regions and the regions at which the first and second busbars 11 and 12 are disposed define high acoustic velocity regions. Accordingly, even though the gap length G is small, a piston mode is provided, and the transverse modes are reduced or prevented. As a result, the propagation loss is reduced. That point will be described below with reference to concrete experimental examples.
- the acoustic wave device 1 was fabricated with the following design parameters.
- the high acoustic velocity support substrate 3 Si substrate
- the low acoustic velocity material layer 4 SiO 2 film having a thickness of about 0.3 ⁇
- the piezoelectric film 5 LiTaO 3 film having Euler Angles (0°, 140°, 0°) and a thickness of about 0.3 ⁇
- Duty of the IDT electrode 6 about 0.45
- the acoustic wave device 1 was fabricated in three configurations with the gap length G set to about 0.17 ⁇ , about 0.28 ⁇ , and about 0.61 ⁇ , for example.
- FIG. 6 plots return loss characteristics of the fabricated acoustic wave devices.
- the return loss characteristics are worst when the gap length G is about 0.61 ⁇ , and are somewhat improved at about 0.28 ⁇ . At about 0.17 ⁇ , the return loss characteristics are most preferable. It is thus understood that the propagation loss is reduced by setting the gap length to about 0.17 ⁇ .
- FIG. 7 represents the evaluation result. Values of the return loss represented in FIG. 7 denote values measured in a range near 2600 MHz where the return loss is the worst.
- the gap length G is about 0.24 ⁇ or longer, the return loss is relatively large.
- the return loss is reduced, and thus the propagation loss is reduced.
- a solid line represents a reverse-velocity surface (the inverse number of SAW-phase velocity) in the multilayer substrate 2
- a dotted line represents a reverse-velocity surface (the inverse number of SAW-phase velocity) in a piezoelectric substrate made of LiTaO 3 alone.
- the reverse-velocity surface (the inverse number of SAW-phase velocity) is convex.
- the reverse-velocity surface (the inverse number of SAW-phase velocity) of the multilayer substrate 2 is different from that of the piezoelectric substrate made of LiTaO 3 alone. Such a difference is presumably the cause of deterioration of the propagation loss when the gap length G is increased. More specifically, the acoustic velocity of a bulk wave due to an SH component in the low acoustic velocity material layer 4 is about 3750 m/s, which is very close to the acoustic velocity of the acoustic wave excited in the IDT electrode 6 . Accordingly, the bulk wave tends to couple with a mode utilized as the acoustic wave. Therefore, when the gap length G is increased, the acoustic wave excited as a main mode leaks in a depth direction of the piezoelectric film 5 . For that reason, the propagation loss deteriorates.
- the gap length G is reduced. Accordingly, it is determined that, as denoted by arrows A in FIG. 8 , the acoustic wave in the main mode tending to leak in the depth direction is reflected by an inner edge of the first busbar 11 , which is positioned closer to the tips of the electrode fingers, and that leakage in the thickness direction is reduced or prevented. As a result, an improvement of the return loss characteristics and a reduction of the propagation loss are achieved.
- FIG. 9 is a partial plan view, partially cut out, illustrating, in an enlarged scale, a principal portion of an electrode structure of an acoustic wave device according to a second preferred embodiment of the present invention.
- the acoustic wave device according to the second preferred embodiment is similar to the acoustic wave device 1 according to the first preferred embodiment except that, in an IDT electrode 26 , structures of the first and second busbars 11 and 12 are different from those in the IDT electrode 6 illustrated in FIG. 1 .
- the openings 31 are arrayed side by side in the propagation direction of the acoustic wave.
- the openings 35 are also arrayed side by side in the propagation direction of the acoustic wave.
- Portions of the first and second busbars 11 and 12 on the side closer to the intersecting region than the openings 31 and 35 define and function as first and second thin busbar portions 32 and 36 , respectively. Portions of the first and second busbars 11 and 12 on the side farther away from the intersecting region than the openings 31 and 35 define and function as first and second outer busbar portions 33 and 37 , respectively.
- the first thin busbar portion 32 and the first outer busbar portion 33 are connected to each other by a plurality of connecting portions 34 .
- the connecting portions 34 are each positioned between the adjacent openings 31 .
- the second thin busbar portion 36 and the second outer busbar portion 37 are connected to each other by a plurality of connecting portions 38 .
- the connecting portion 34 is disposed along an extension of the first electrode finger 13 in a direction outward from the intersecting region.
- the connecting portion 38 is disposed at a position along an extension of the second electrode finger 14 in a direction outward from the intersecting region.
- the gap lengths between the tips of the first and second electrode fingers 13 , 14 and the second and first busbars 12 , 11 are preferably set to about 0.23 ⁇ or shorter, for example.
- the propagation loss is reduced as in the first preferred embodiment.
- the acoustic velocities in various regions crossing perpendicularly or substantially perpendicularly to the propagation direction of the acoustic wave in the IDT electrode 26 are as shown in FIG. 9 .
- the acoustic velocity in the central region is denoted by V 1
- the acoustic velocity in the first and second edge regions is denoted by V 2
- the acoustic velocity in the first and second gap regions is denoted by V 10
- the acoustic velocity in the first and second thin busbar portions 32 and 36 is denoted by V 11
- the acoustic velocity in regions at which the openings 31 and 35 are provided is denoted by V 12
- the acoustic velocity in regions at which the first and second outer busbar portions 33 and 37 are disposed is denoted by V 13 .
- the second or first gap and the first or second thin busbar portion 32 or 36 provide higher acoustic velocity regions in comparison with the first or second edge region. As a result, transverse mode ripples are reduced or prevented as in the first preferred embodiment.
- FIG. 10 is a plan view illustrating an electrode structure of an acoustic wave device according to a third preferred embodiment of the present invention.
- an IDT electrode 41 is a normal IDT electrode.
- a first busbar 42 and a second busbar 43 are opposed to each other.
- One end of each of a plurality of first electrode fingers 44 is connected to the first busbar 42 .
- One end of each of a plurality of second electrode fingers 45 is connected to the second busbar 43 .
- the first and second electrode fingers 44 and 45 have a constant or substantially constant width over the entire length.
- the acoustic wave device may preferably include the normal IDT electrode.
- gap lengths G between tips of the first and second electrode fingers 44 , 45 and the second and first busbars 43 , 42 are preferably set to about 0.23 ⁇ or shorter, for example.
- the propagation loss is reduced as in the first and second preferred embodiments.
- FIG. 11 is a plan view to explain an IDT electrode 51 in an acoustic wave device according to a fourth preferred embodiment of the present invention.
- a plurality of first electrode fingers 52 and a plurality of second electrode fingers 53 are laminated in the IDT electrode 51 .
- the first electrode fingers 52 and the second electrode fingers 53 have a constant or substantially constant width.
- first dielectric films 54 defining and functioning as mass adding films are laminated on the first and second electrode fingers in the first and second edge regions thereof.
- the first dielectric film 54 is preferably made of a dielectric such as silicon oxide, tantalum pentoxide, or tellurium oxide, for example.
- the acoustic velocity is reduced in regions at which the first dielectric films 54 are laminated on the first and second electrode fingers 52 and 53 .
- the first dielectric films 54 extend in the propagation direction of the acoustic wave.
- the first dielectric films 54 are laminated on the piezoelectric film 5 as well as between the first and second electrode fingers 52 and 53 .
- FIG. 12 is a plan view to explain an IDT electrode 61 in an acoustic wave device according to a fifth preferred embodiment of the present invention.
- metal films 62 defining and functioning as mass adding films are laminated on portions of the first and second electrode fingers 52 and 53 , those portions being positioned in the first and second edge regions.
- the acoustic velocity is reduced in the first and second edge regions.
- FIG. 13 is a schematic side sectional view of an acoustic wave device 71 according to a sixth preferred embodiment of the present invention.
- first dielectric films 54 A are laminated under an IDT electrode 51 A. More specifically, the first dielectric films 54 A are laminated under the first electrode fingers 52 in the first and second edge regions thereof. Thus, the first dielectric films 54 A may preferably be disposed under the IDT electrode 51 A in an opposite positional relationship to the above-described first dielectric films 54 .
- a second dielectric film 72 covers the IDT electrode 51 A. With the second dielectric film 72 , it is possible to protect the IDT electrode 51 A and to perform frequency adjustment. In other words, the frequency adjustment is performed by adjusting a thickness and a material of the second dielectric film 72 .
- the propagation loss is reduced as in the first preferred embodiment preferably by setting the gap length G to about 0.23 ⁇ or shorter, for example.
- FIG. 14 is a plan view to explain an IDT electrode in an acoustic wave device according to a seventh preferred embodiment of the present invention.
- FIG. 15 is a schematic side sectional view of the acoustic wave device illustrated in FIG. 14 .
- first dielectric films 54 are laminated on the IDT electrode 51 . While, in FIG. 11 , the first dielectric films 54 are disposed in the first and second edge regions, the first dielectric films 54 are laminated, in the seventh preferred embodiment illustrated in FIG. 14 , so as to cover not only the first and second edge regions, but also the gap regions and the first and second busbars. Also in such a case, the acoustic velocities in the first and second edge regions are reduced with the lamination of the first dielectric films 54 .
- the acoustic velocity in the central region is denoted by V 1
- the acoustic velocity in the first and second edge regions is denoted by V 2
- the acoustic velocity in the gap regions is denoted by V 3
- the acoustic velocity in regions at which the first and second busbars are disposed is denoted by V 4
- those acoustic velocities preferably satisfy a relationship of V 3 >V 1 ⁇ V 4 >V 2 .
- the propagation loss is reduced preferably by setting the gap length to about 0.23 ⁇ or shorter, for example.
- FIG. 16 is a side sectional view of an acoustic wave device according to an eighth preferred embodiment of the present invention.
- a second dielectric film 72 defining and functioning as a frequency adjustment film is laminated between the first dielectric film 54 and the IDT electrode 51 .
- the second dielectric film 72 is preferably made of an appropriate dielectric such as silicon oxide, for example. With the second dielectric film 72 , frequency adjustment is able to be performed. Thus, the second dielectric film 72 may preferably be disposed between the first dielectric film 54 and the IDT electrode 51 .
- the first dielectric films 54 are laminated on the IDT electrode 51 in FIG. 15 , the first dielectric films 54 may be disposed under the IDT electrode 51 as illustrated in FIG. 17 .
- the second dielectric film 72 defining and functioning as the frequency adjustment film may cover the IDT electrode 51 .
- the second dielectric film 72 may cover the IDT electrode 51 , and the first dielectric films 54 may be embedded in the second dielectric film 72 .
- the propagation loss is reduced preferably when the gap length G is reduced to about 0.23 ⁇ or shorter, for example.
- FIG. 19 is a graph representing relationships between the length of the edge region and the electromechanical coupling coefficient in transverse modes in the acoustic wave device including the IDT electrode illustrated in FIG. 11 .
- the length of the edge region is a length of each of the first and second edge regions in the extension direction of the first and second electrode fingers.
- FIG. 20 is a graph representing relationships between the length of the edge region and the electromechanical coupling coefficient in transverse modes in the acoustic wave device including the IDT electrode illustrated in FIG. 14 .
- Each of FIGS. 19 and 20 represents the result for the third, fifth, seventh, and ninth transverse modes. Compared to the result plotted in FIG.
- the ability to reduce or prevent the transverse modes with respect to variations in tip size of the IDT electrode 51 is higher in the result plotted in FIG. 20 .
- the first dielectric films 54 preferably cover not only the first and second edge regions, but also the gap regions and the regions at which the first and second busbars are disposed, as illustrated in FIG. 14 .
- FIG. 21 is a front sectional view of an acoustic wave device 91 according to a ninth preferred embodiment of the present invention.
- the acoustic wave device 91 includes a multilayer substrate 2 A.
- a high acoustic velocity material layer 3 B, a low acoustic velocity material layer 4 , and a piezoelectric film 5 preferably made of LiTaO 3 , for example, are laminated on a support substrate 3 A.
- the support substrate 3 A may preferably be made of a material other than the high acoustic velocity material.
- the support substrate 3 A may be made of the high acoustic velocity material.
- the acoustic wave device 91 is similar to the acoustic wave device 1 according to the first preferred embodiment, except that the multilayer substrate 2 A has a different structure from the multilayer substrate 2 illustrated in FIG. 2 . Therefore, the propagation loss is reduced in the acoustic wave device 71 according to the ninth preferred embodiment as well.
- the thickness of the piezoelectric film 5 is preferably about 3.5 ⁇ or smaller, for example.
- Q characteristics degrade when the film thickness of LiTaO 3 exceeds about 3.5 ⁇ .
- the thickness of LiTaO 3 is preferably about 3.5 ⁇ or smaller, for example, so as to improve the Q characteristics.
- the above-described high acoustic velocity material may preferably be selected from among semiconductors such as Si, inorganic insulators such as SiN and Al 2 O 3 , metals such as Pt and W, and alloys of those metals, for example.
- the above-described low acoustic velocity material may preferably be selected from among inorganic insulating materials such as silicon oxide and silicon oxynitride, and resin materials, for example.
- the present invention may be further applied to acoustic wave devices including other electrode structures, such as an acoustic wave filter, as long as the IDT electrode has the above-described structure.
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Abstract
Description
- This application claims the benefit of priority to Japanese Patent Application No. 2017-203112 filed on Oct. 20, 2017. The entire contents of this application are hereby incorporated herein by reference.
- The present invention relates to an acoustic wave device including a multilayer substrate in which a high acoustic velocity material layer, a low acoustic velocity material layer, and a piezoelectric film are laminated.
- An IDT (interdigital transducer) electrode of an acoustic wave device includes first and second busbars opposing to each other, a plurality of first electrode fingers, and a plurality of second electrode fingers. One end of each of the first electrode fingers is connected to the first busbar. One end of each of the second electrode fingers is connected to the second busbar. Tips of the first electrode fingers are arranged with gaps interposed between the tips and the second busbar. Tips of the second electrode fingers are arranged with gaps interposed between the tips and the first busbar. Japanese Unexamined Patent Application Publication No. 2011-101350 discloses that wave guide is performed when a length of the gaps is 1 λ or longer, and that the gap length is more desirably 3 λ or longer.
- As disclosed in Japanese Unexamined Patent Application Publication No. 2011-101350, it has been considered that the gap between the tip of the electrode finger and the busbar desirably has a longer length.
- However, the inventor of preferred embodiments of the present invention has discovered that a propagation loss increases at a longer gap length in an acoustic wave device in which a piezoelectric substrate is a multilayer substrate. In particular, a propagation loss tends to increase in a multilayer substrate in which a high acoustic velocity material layer, a low acoustic velocity material layer, and a piezoelectric film are laminated. The term “high acoustic velocity material” denotes a material in which the acoustic velocity of a bulk wave propagating in the material is higher than the propagation velocity of an acoustic wave propagating in the piezoelectric film. The term “low acoustic velocity material” denotes a material in which the acoustic velocity of a bulk wave propagating in the material is lower than the propagation velocity of the bulk wave propagating in the piezoelectric film.
- Preferred embodiments of the present invention provide acoustic wave devices each including a multilayer substrate and having a reduced propagation loss.
- According to a preferred embodiment of the present invention, an acoustic wave device includes a multilayer substrate including a reverse-velocity surface (the inverse of SAW-phase velocity), the multilayer substrate including a piezoelectric film, a low acoustic velocity material layer in which a bulk wave having a lower velocity than an acoustic velocity of an acoustic wave propagating in the piezoelectric film propagates, and a high acoustic velocity material layer in which a bulk wave having higher velocity than the acoustic velocity of the acoustic wave propagating in the piezoelectric film propagates, the low acoustic velocity material layer being disposed between the high acoustic velocity material layer and the piezoelectric film; and an IDT electrode disposed on the piezoelectric film, wherein the IDT electrode includes first and second busbars opposing each other, a plurality of first electrode fingers extending from the first busbar toward the second busbar, and a plurality of second electrode fingers extending from the second busbar toward the first busbar, and wherein gap lengths of a first gap between a tip of each of the first electrode fingers and the second busbar and a second gap between a tip of each of the second electrode fingers and the first busbar are about 0.23 λ or shorter, the gap lengths being measured in an extension direction of the first and second electrode fingers.
- In an acoustic wave device according to a preferred embodiment of the present invention, in the IDT electrode, an intersecting region in which the first electrode fingers and the second electrode fingers are overlapped with each other in a propagation direction of the acoustic wave includes a central region at a middle in the extension direction of the first and second electrode fingers, and first and second edge regions on both of the outer sides of the central region in the extension direction of the first and second electrode fingers, and the acoustic velocity in each of the first and second edge regions is lower than the acoustic velocity in the central region. In such a case, transverse mode ripples are reduced or prevented.
- In an acoustic wave device according to a preferred embodiment of the present invention, widths of the first and second electrode fingers in the first and second edge regions are greater than widths of the first and second electrode fingers in the central region, the widths being measured in the propagation direction of the acoustic wave.
- In an acoustic wave device according to a preferred embodiment of the present invention, each of the first and second busbars includes a plurality of openings arrayed side by side in the propagation direction of the acoustic wave, portions of the first and second busbars on a side closer to the intersecting region than the openings define and function as first and second thin busbar portions, and portions of the first and second busbars on a side farther away from the intersecting region than the openings define and function as first and second outer busbar portions, the first and second thin busbar portions being connected respectively to the first and second outer busbar portions by connecting portions between the adjacent openings.
- In an acoustic wave device according to a preferred embodiment of the present invention, the piezoelectric film is made of LiTaO3 and has a thickness of about 3.5 λ or smaller. In such a case, a Q value is increased.
- In an acoustic wave device according to a preferred embodiment of the present invention, the high acoustic velocity material layer may be a support substrate made of a high acoustic velocity material.
- The acoustic wave device according to a preferred embodiment of the present invention may further include a support substrate that supports the high acoustic velocity material layer.
- In an acoustic wave device according to a preferred embodiment of the present invention, first dielectric films that define mass adding films are laminated on the first and second edge regions.
- In an acoustic wave device according to a preferred embodiment of the present invention, metal films that define mass adding films are laminated on portions of the first and second electrode fingers positioned in the first and second edge regions.
- In an acoustic wave device according to a preferred embodiment of the present invention, the first dielectric films are further laminated on the first and second busbars and over the first and second gaps.
- In an acoustic wave device according to a preferred embodiment of the present invention, a second dielectric film that defines a frequency adjustment film is laminated on the IDT electrode, and the first dielectric films are laminated with the second dielectric film interposed between the IDT electrode and the first dielectric films.
- According to preferred embodiments of the present invention, propagation losses are able to be reduced or prevented in the acoustic wave devices including the multilayer substrate in which the high acoustic velocity material layer, the low acoustic velocity material layer, and the piezoelectric film are laminated.
- The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.
-
FIG. 1 is a partial plan view illustrating a principal portion of an electrode structure of an acoustic wave device according to a first preferred embodiment of the present invention. -
FIG. 2 is a schematic front sectional view of the acoustic wave device according to the first preferred embodiment of the present invention. -
FIG. 3 is a schematic plan view illustrating the electrode structure of the acoustic wave device according to the first preferred embodiment of the present invention. -
FIG. 4 is a partial enlarged plan view, partly cut out, illustrating a portion ofFIG. 1 in an enlarged scale. -
FIG. 5 is an illustration representing reverse-velocity surfaces (the inverse number of SAW-phase velocity) of LiTaO3 and a multilayer substrate included in the first preferred embodiment of the present invention. -
FIG. 6 is a graph representing return loss characteristics of the acoustic wave device in which a gap length G is about 0.17 λ, about 0.28 λ, or about 0.61 λ. -
FIG. 7 is a graph representing a relationship between the gap length G in an IDT electrode and return loss. -
FIG. 8 is a schematic view to explain the reason why a propagation loss reduces in the acoustic wave device according to the first preferred embodiment of the present invention. -
FIG. 9 is a partial plan view, partially cut out, illustrating an electrode structure of an acoustic wave device according to a second preferred embodiment of the present invention, and acoustic velocities in various regions. -
FIG. 10 is a plan view to explain an electrode structure of an acoustic wave device according to a third preferred embodiment of the present invention. -
FIG. 11 is a plan view to explain an IDT electrode in an acoustic wave device according to a fourth preferred embodiment of the present invention. -
FIG. 12 is a plan view to explain an electrode structure of an IDT electrode in an acoustic wave device according to a fifth preferred embodiment of the present invention. -
FIG. 13 is a schematic side sectional view of an acoustic wave device according to a sixth preferred embodiment of the present invention. -
FIG. 14 is a plan view to explain an electrode structure of an IDT electrode in an acoustic wave device according to a seventh preferred embodiment of the present invention. -
FIG. 15 is a schematic side sectional view of the acoustic wave device according to the seventh preferred embodiment of the present invention. -
FIG. 16 is a schematic side sectional view of an acoustic wave device according to an eighth preferred embodiment of the present invention. -
FIG. 17 is a schematic side sectional view to explain a modification of the acoustic wave device according to the eighth preferred embodiment of the present invention. -
FIG. 18 is a schematic side sectional view to explain another modification of the acoustic wave device according to the eighth preferred embodiment of the present invention. -
FIG. 19 is a graph representing relationships between the length of an edge region and the electromechanical coupling coefficient in transverse modes in the acoustic wave device according to the fourth preferred embodiment of the present invention. -
FIG. 20 is a graph representing relationships between the length of an edge region and the electromechanical coupling coefficient in transverse modes in the acoustic wave device according to the seventh preferred embodiment of the present invention. -
FIG. 21 is a front sectional view of an acoustic wave device according to a ninth preferred embodiment of the present invention. -
FIG. 22 is a graph representing a relationship between a film thickness of LiTaO3 and Q characteristics. - Preferred embodiments of the present invention will be described with reference to the drawings.
- It is to be noted that the preferred embodiments described in this specification are merely illustrative, and that partial replacements and combinations of individual components may be made between the different preferred embodiments.
-
FIG. 2 is a schematic front sectional view of anacoustic wave device 1 according to a first preferred embodiment of the present invention, andFIG. 3 is a schematic plan view illustrating an electrode structure of the acoustic wave device. - The
acoustic wave device 1 includes amultilayer substrate 2. Themultilayer substrate 2 includes a high acousticvelocity support substrate 3 defining a high acoustic velocity material layer, a low acousticvelocity material layer 4, and apiezoelectric film 5 preferably made of LiTaO3, for example. The low acousticvelocity material layer 4 and thepiezoelectric film 5 are laminated on the high acousticvelocity support substrate 3 in that order. AnIDT electrode 6 andreflectors piezoelectric film 5. A one-port acoustic wave resonator is thus provided. - A high acoustic velocity material defining the high acoustic velocity material layer is a material such that the acoustic velocity of a bulk wave propagating in the material is higher than the acoustic velocity of an acoustic wave propagating in the
piezoelectric film 5. A low acoustic velocity material defining the low acousticvelocity material layer 4 is a material such that the acoustic velocity of a bulk wave propagating in the material is lower than the acoustic velocity of the acoustic wave propagating in thepiezoelectric film 5. In themultilayer substrate 2 including the low acousticvelocity material layer 4 and thepiezoelectric film 5 laminated on the high acousticvelocity support substrate 3, energy of the acoustic wave is effectively confined inside thepiezoelectric film 5. - In this connection, as described above, the inventor of preferred embodiments of the present invention discovered that, when the
multilayer substrate 2 is used, a propagation loss increases as a gap size in theIDT electrode 6 increases. - As illustrated in
FIG. 3 , theIDT electrode 6 includes first andsecond busbars first electrode fingers 13 is connected to thefirst busbar 11. Thefirst electrode fingers 13 extend toward thesecond busbar 12. One end of each of a plurality ofsecond electrode fingers 14 is connected to thesecond busbar 12. Thesecond electrode fingers 14 extend toward thefirst busbar 11. - A space between a tip of each
first electrode finger 13 and thesecond busbar 12 is a first gap. A space between a tip of eachsecond electrode finger 14 and thefirst busbar 11 is a second gap. Lengths of the first and second gaps in an extension direction of the first andsecond electrode fingers - In the
acoustic wave device 1, the gap length G is preferably set to about 0.23 λ or shorter, for example, such that the propagation loss is able to be reduced. -
FIG. 3 schematically illustrates theIDT electrode 6, whileFIGS. 1 and 4 illustrate a principal portion of theIDT electrode 6 in an enlarged scale. In addition,FIG. 4 further indicates the acoustic velocities in various regions. Each of thefirst electrode fingers 13 includesgreater width portions second electrode fingers 14 also includesgreater width portions greater width portions second electrode fingers greater width portion 13 b is disposed at a position opposing thegreater width portion 14 a in the propagation direction of the acoustic wave. Thegreater width portion 14 b is disposed in an opposing relationship to thegreater width portion 13 a in the propagation direction of the acoustic wave. - Accordingly, an intersecting region C, i.e., a region at which the
first electrode fingers 13 and thesecond electrode fingers 14 are overlapped with each other in the propagation direction of the acoustic wave, includes a central region C1, and first and second edge regions C2 and C3 on both the outer sides of the central region C1 in a direction perpendicular or substantially perpendicular to the propagation direction of the acoustic wave. Furthermore, a space on the outer side of the first edge region C2 provides a gap region at which the second gap is provided, the gap region being called a “second gap region”. A space on the outer side of the second edge region C3 in the direction perpendicular or substantially perpendicular to the propagation direction of the acoustic wave provides a first gap region at which the first gap is provided. Regions on the outer sides of the first and second gap regions provide regions at which thesecond busbar 12 and thefirst busbar 11 are provided, respectively.FIG. 4 diagrammatically indicates the acoustic velocities in the above-described regions. Regarding the acoustic velocities in the above-described regions illustrated inFIG. 4 , the acoustic velocity is higher toward the right inFIG. 4 . - In the
acoustic wave device 1, theIDT electrode 6 is designed as follows. The acoustic velocity V2 in the first and second edge regions C2 and C3 is lower than the acoustic velocity V1 in the central region C1. The gap regions having the acoustic velocity V3 are provided on the outer sides of the first and second edge regions C2 and C3. In addition, the acoustic velocity in the regions at which the first andsecond busbars - In the case of using the multilayer substrate, transverse modes cause a problem. By designing the
IDT electrode 6 to satisfy the above relationship among the acoustic velocities, the transverse modes are reduced or prevented due to the difference in the acoustic velocity. It is, however, to be noted that the present invention is not limited to acoustic wave devices including electrode structures that reduce or prevent the transverse modes. - As described above, the
acoustic wave device 1 includes the gap length G that is preferably about 0.23 λ or shorter, for example. In theacoustic wave device 1, the gap regions and the regions at which the first andsecond busbars - The
acoustic wave device 1 was fabricated with the following design parameters. - Structure of the
multilayer substrate 2; the high acoustic velocity support substrate 3: Si substrate, the low acoustic velocity material layer 4: SiO2 film having a thickness of about 0.3 λ, the piezoelectric film 5: LiTaO3 film having Euler Angles (0°, 140°, 0°) and a thickness of about 0.3 λ - Materials of the
IDT electrode 6 and thereflectors 7 and 8: Al - Number of pairs of the electrode fingers of the IDT electrode 6: 100 pairs
- Duty of the IDT electrode 6: about 0.45
- Number of the electrode fingers of the
reflectors 7 and 8: 20 pairs - Film thicknesses of the
IDT electrode 6 and thereflectors 7 and 8: about 0.05 λ, λ=about 2 μm - The
acoustic wave device 1 was fabricated in three configurations with the gap length G set to about 0.17 λ, about 0.28 λ, and about 0.61 λ, for example.FIG. 6 plots return loss characteristics of the fabricated acoustic wave devices. - As shown in
FIG. 6 , the return loss characteristics are worst when the gap length G is about 0.61 λ, and are somewhat improved at about 0.28 λ. At about 0.17 λ, the return loss characteristics are most preferable. It is thus understood that the propagation loss is reduced by setting the gap length to about 0.17 λ. - Taking the above-described result into account, the return loss characteristics were evaluated while the gap length G was more finely changed in the
acoustic wave device 1.FIG. 7 represents the evaluation result. Values of the return loss represented inFIG. 7 denote values measured in a range near 2600 MHz where the return loss is the worst. - As shown in
FIG. 7 , when the gap length G is about 0.24 λ or longer, the return loss is relatively large. On the other hand, at about 0.23 λ or shorter, the return loss is reduced, and thus the propagation loss is reduced. - The reason that the gap length G is to be reduced as described above is presumably as follows. In
FIG. 5 , a solid line represents a reverse-velocity surface (the inverse number of SAW-phase velocity) in themultilayer substrate 2, and a dotted line represents a reverse-velocity surface (the inverse number of SAW-phase velocity) in a piezoelectric substrate made of LiTaO3 alone. As seen fromFIG. 5 , in the case of using themultilayer substrate 2, the reverse-velocity surface (the inverse number of SAW-phase velocity) is convex. Furthermore, the reverse-velocity surface (the inverse number of SAW-phase velocity) of themultilayer substrate 2 is different from that of the piezoelectric substrate made of LiTaO3 alone. Such a difference is presumably the cause of deterioration of the propagation loss when the gap length G is increased. More specifically, the acoustic velocity of a bulk wave due to an SH component in the low acousticvelocity material layer 4 is about 3750 m/s, which is very close to the acoustic velocity of the acoustic wave excited in theIDT electrode 6. Accordingly, the bulk wave tends to couple with a mode utilized as the acoustic wave. Therefore, when the gap length G is increased, the acoustic wave excited as a main mode leaks in a depth direction of thepiezoelectric film 5. For that reason, the propagation loss deteriorates. - On the other hand, in the
acoustic wave device 1, the gap length G is reduced. Accordingly, it is determined that, as denoted by arrows A inFIG. 8 , the acoustic wave in the main mode tending to leak in the depth direction is reflected by an inner edge of thefirst busbar 11, which is positioned closer to the tips of the electrode fingers, and that leakage in the thickness direction is reduced or prevented. As a result, an improvement of the return loss characteristics and a reduction of the propagation loss are achieved. -
FIG. 9 is a partial plan view, partially cut out, illustrating, in an enlarged scale, a principal portion of an electrode structure of an acoustic wave device according to a second preferred embodiment of the present invention. The acoustic wave device according to the second preferred embodiment is similar to theacoustic wave device 1 according to the first preferred embodiment except that, in anIDT electrode 26, structures of the first andsecond busbars IDT electrode 6 illustrated inFIG. 1 . - In the
IDT electrode 26, a plurality ofopenings 31 and are provided in the first andsecond busbars openings 31 are arrayed side by side in the propagation direction of the acoustic wave. Theopenings 35 are also arrayed side by side in the propagation direction of the acoustic wave. - Portions of the first and
second busbars openings thin busbar portions second busbars openings outer busbar portions thin busbar portion 32 and the firstouter busbar portion 33 are connected to each other by a plurality of connectingportions 34. The connectingportions 34 are each positioned between theadjacent openings 31. - Similarly, in the
second busbar 12, the secondthin busbar portion 36 and the secondouter busbar portion 37 are connected to each other by a plurality of connectingportions 38. - The connecting
portion 34 is disposed along an extension of thefirst electrode finger 13 in a direction outward from the intersecting region. The connectingportion 38 is disposed at a position along an extension of thesecond electrode finger 14 in a direction outward from the intersecting region. - Also in the acoustic wave device according to the second preferred embodiment, the gap lengths between the tips of the first and
second electrode fingers first busbars - In the acoustic wave device according to the second preferred embodiment, the acoustic velocities in various regions crossing perpendicularly or substantially perpendicularly to the propagation direction of the acoustic wave in the
IDT electrode 26 are as shown inFIG. 9 . - The acoustic velocity in the central region is denoted by V1, and that the acoustic velocity in the first and second edge regions is denoted by V2. In addition, the acoustic velocity in the first and second gap regions is denoted by V10, the acoustic velocity in the first and second
thin busbar portions openings outer busbar portions - In the above-described case, the second or first gap and the first or second
thin busbar portion -
FIG. 10 is a plan view illustrating an electrode structure of an acoustic wave device according to a third preferred embodiment of the present invention. In the acoustic wave device according to the third preferred embodiment, anIDT electrode 41 is a normal IDT electrode. Afirst busbar 42 and asecond busbar 43 are opposed to each other. One end of each of a plurality offirst electrode fingers 44 is connected to thefirst busbar 42. One end of each of a plurality ofsecond electrode fingers 45 is connected to thesecond busbar 43. Furthermore, the first andsecond electrode fingers - Also in the present preferred embodiment, gap lengths G between tips of the first and
second electrode fingers first busbars -
FIG. 11 is a plan view to explain anIDT electrode 51 in an acoustic wave device according to a fourth preferred embodiment of the present invention. In the acoustic wave device according to the fourth preferred embodiment, a plurality offirst electrode fingers 52 and a plurality ofsecond electrode fingers 53 are laminated in theIDT electrode 51. Thefirst electrode fingers 52 and thesecond electrode fingers 53 have a constant or substantially constant width. Moreover, firstdielectric films 54 defining and functioning as mass adding films are laminated on the first and second electrode fingers in the first and second edge regions thereof. - The
first dielectric film 54 is preferably made of a dielectric such as silicon oxide, tantalum pentoxide, or tellurium oxide, for example. The acoustic velocity is reduced in regions at which the firstdielectric films 54 are laminated on the first andsecond electrode fingers dielectric films 54 extend in the propagation direction of the acoustic wave. Thus, the firstdielectric films 54 are laminated on thepiezoelectric film 5 as well as between the first andsecond electrode fingers -
FIG. 12 is a plan view to explain anIDT electrode 61 in an acoustic wave device according to a fifth preferred embodiment of the present invention. In theIDT electrode 61,metal films 62 defining and functioning as mass adding films are laminated on portions of the first andsecond electrode fingers -
FIG. 13 is a schematic side sectional view of anacoustic wave device 71 according to a sixth preferred embodiment of the present invention. In theacoustic wave device 71, firstdielectric films 54A are laminated under anIDT electrode 51A. More specifically, the firstdielectric films 54A are laminated under thefirst electrode fingers 52 in the first and second edge regions thereof. Thus, the firstdielectric films 54A may preferably be disposed under theIDT electrode 51A in an opposite positional relationship to the above-described firstdielectric films 54. Furthermore, in theacoustic wave device 71, asecond dielectric film 72 covers theIDT electrode 51A. With thesecond dielectric film 72, it is possible to protect theIDT electrode 51A and to perform frequency adjustment. In other words, the frequency adjustment is performed by adjusting a thickness and a material of thesecond dielectric film 72. - Also in the acoustic wave devices according to the fourth to sixth preferred embodiments, the propagation loss is reduced as in the first preferred embodiment preferably by setting the gap length G to about 0.23 λ or shorter, for example.
-
FIG. 14 is a plan view to explain an IDT electrode in an acoustic wave device according to a seventh preferred embodiment of the present invention.FIG. 15 is a schematic side sectional view of the acoustic wave device illustrated inFIG. 14 . - In the seventh preferred embodiment, first
dielectric films 54 are laminated on theIDT electrode 51. While, inFIG. 11 , the firstdielectric films 54 are disposed in the first and second edge regions, the firstdielectric films 54 are laminated, in the seventh preferred embodiment illustrated inFIG. 14 , so as to cover not only the first and second edge regions, but also the gap regions and the first and second busbars. Also in such a case, the acoustic velocities in the first and second edge regions are reduced with the lamination of the firstdielectric films 54. Furthermore, in the seventh preferred embodiment, the acoustic velocity in the central region is denoted by V1, the acoustic velocity in the first and second edge regions is denoted by V2, the acoustic velocity in the gap regions is denoted by V3, and the acoustic velocity in regions at which the first and second busbars are disposed is denoted by V4, those acoustic velocities preferably satisfy a relationship of V3>V1≈V4>V2. Also in the present preferred embodiment, therefore, the propagation loss is reduced preferably by setting the gap length to about 0.23 λ or shorter, for example. -
FIG. 16 is a side sectional view of an acoustic wave device according to an eighth preferred embodiment of the present invention. In the acoustic wave device according to the eighth preferred embodiment, asecond dielectric film 72 defining and functioning as a frequency adjustment film is laminated between thefirst dielectric film 54 and theIDT electrode 51. Thesecond dielectric film 72 is preferably made of an appropriate dielectric such as silicon oxide, for example. With thesecond dielectric film 72, frequency adjustment is able to be performed. Thus, thesecond dielectric film 72 may preferably be disposed between thefirst dielectric film 54 and theIDT electrode 51. - Furthermore, while the first
dielectric films 54 are laminated on theIDT electrode 51 inFIG. 15 , the firstdielectric films 54 may be disposed under theIDT electrode 51 as illustrated inFIG. 17 . In such a case, thesecond dielectric film 72 defining and functioning as the frequency adjustment film may cover theIDT electrode 51. - As illustrated in
FIG. 18 , thesecond dielectric film 72 may cover theIDT electrode 51, and the firstdielectric films 54 may be embedded in thesecond dielectric film 72. - Also with the structures illustrated in
FIGS. 14 to 18 , since the gap regions provide the high acoustic velocity regions, the propagation loss is reduced preferably when the gap length G is reduced to about 0.23 λ or shorter, for example. -
FIG. 19 is a graph representing relationships between the length of the edge region and the electromechanical coupling coefficient in transverse modes in the acoustic wave device including the IDT electrode illustrated inFIG. 11 . The length of the edge region is a length of each of the first and second edge regions in the extension direction of the first and second electrode fingers.FIG. 20 is a graph representing relationships between the length of the edge region and the electromechanical coupling coefficient in transverse modes in the acoustic wave device including the IDT electrode illustrated inFIG. 14 . Each ofFIGS. 19 and 20 represents the result for the third, fifth, seventh, and ninth transverse modes. Compared to the result plotted inFIG. 19 , the ability to reduce or prevent the transverse modes with respect to variations in tip size of theIDT electrode 51 is higher in the result plotted inFIG. 20 . For that reason, the firstdielectric films 54 preferably cover not only the first and second edge regions, but also the gap regions and the regions at which the first and second busbars are disposed, as illustrated inFIG. 14 . -
FIG. 21 is a front sectional view of anacoustic wave device 91 according to a ninth preferred embodiment of the present invention. Theacoustic wave device 91 includes a multilayer substrate 2A. In the multilayer substrate 2A, a high acoustic velocity material layer 3B, a low acousticvelocity material layer 4, and apiezoelectric film 5 preferably made of LiTaO3, for example, are laminated on asupport substrate 3A. Because the high acoustic velocity material layer 3B is provided, thesupport substrate 3A may preferably be made of a material other than the high acoustic velocity material. Of course, thesupport substrate 3A may be made of the high acoustic velocity material. - The
acoustic wave device 91 is similar to theacoustic wave device 1 according to the first preferred embodiment, except that the multilayer substrate 2A has a different structure from themultilayer substrate 2 illustrated inFIG. 2 . Therefore, the propagation loss is reduced in theacoustic wave device 71 according to the ninth preferred embodiment as well. - When the
multilayer substrates 2 and 2A illustrated inFIGS. 2 and 21 are used, the thickness of thepiezoelectric film 5 is preferably about 3.5 λ or smaller, for example. As plotted inFIG. 22 , when themultilayer substrate 2 is used, Q characteristics degrade when the film thickness of LiTaO3 exceeds about 3.5 λ. Thus, the thickness of LiTaO3 is preferably about 3.5 λ or smaller, for example, so as to improve the Q characteristics. - The above-described high acoustic velocity material may preferably be selected from among semiconductors such as Si, inorganic insulators such as SiN and Al2O3, metals such as Pt and W, and alloys of those metals, for example.
- The above-described low acoustic velocity material may preferably be selected from among inorganic insulating materials such as silicon oxide and silicon oxynitride, and resin materials, for example.
- It is to be noted that appropriate materials may be used as the high acoustic velocity material and the low acoustic velocity material in proper combinations as long as the above-described relationship in the acoustic velocity is satisfied.
- While the first to ninth preferred embodiments have been described in connection with the one-port acoustic wave resonator, the present invention may be further applied to acoustic wave devices including other electrode structures, such as an acoustic wave filter, as long as the IDT electrode has the above-described structure.
- While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.
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